The present invention relates to an electrographic process including an electrophotographic process for the formation of conventional toner images, digital color images and transparent color images by a digital color copying apparatus, or a digital color printer (color image processor).
As a heat application roller for fixing toner images formed by electrophotography to a recording sheet under application of heat thereto, for example, a roller coated with a silicone elastomer and a roller coated with a fluorine-containing resin having a toner-releasable property have been proposed. The resins for the toners for use with such rollers are proposed, for example, in Japanese Patent Publication 55-6895, Japanese Laid-Open Patent Application 50-44836, Japanese Patent Publication 46-12680, and Japanese Laid-Open Patent Application 51-147325.
When full color images are formed by the toners comprising the above resins and fixed by the conventional image fixing method, the above toners have the following drawbacks:
(1) When the toners comprising a styrene-acrylic resin are used, glossy images are hardly obtained, so that they are not suitable for the formation of color images, although the toners do not cause any disadvantageous offset phenomenon. When the fixed images are not glossy, the light reflected by the surface of the images is scattered, since the inner reflection light having a particular color and the scattered reflection light are simultaneously sensed by the eyes, the density of the visible color is decreased and therefore the images look light-colored or faint and are not impressive.
(2) Since the toners comprising a resin having a relatively low molecular weight, such as polyester and epoxy resin, are softened at low temperatures, glossy images can be easily obtained and therefore such toners are suitable for color copying. However, the toners tend to adhere to an image fixing roller and are poor in the releasability from the roller, so that the offset phenomenon is apt to occur. Therefore it is necessary to apply an auxiliary agent such as silicone oil to an silicone-elastomer-coated roller with improved releasability. A Teflon roller does not have a sufficient image fixing temperature range for use in practice.
In a conventional full color copying apparatus, toners of a low-temperature-softening type, and a silicone-oil-applied silicone roller are employed for image fixing. However, the roller with improved releasability has the shortcoming that its life is not long. In order to solve this problem, it may be expected that an image fixing roller made of a fluorine-containing resin will be useful. However, the fact is that a roller made of the fluorine-containing resin is inferior in the toner releasability to the silicone roller, and when a low-temperature-softening type toner is used with the roller made of the fluorine-containing resin, the offset phenomenon is apt to occur and the image fixable temperature range is narrow.
For color hard copies obtained by digital printing, a reproduction method of an area-gradation type is employed as a pseudo-color image gradation method. However, in the above digital printing, since the toners mentioned in the above (2) are employed in combination with the silicone-elastomer-coated roller with improved releasability, glossy images can be obtained. However, reproduction of dot images is not so good, possibly because the melted toner flows into the fibers of the paper. The result is that it is difficult to obtain images with the desired gradation and quality.